This invention relates to a method for recovering certain water-soluble organotin halides from aqueous solutions containing the organotin compounds.
Organotin halides of the general formula R.sub.a SnX.sub.4.sub.-a wherein each R represents a hydrocarbon radical, particularly a lower alkyl radical and X represents chlorine, bromine or iodine, and a is 1, 2 or 3 are employed as biocides for controlling a variety of undesirable organisms and as auxiliary heat stabilizers for halogen-containing polymers. A major portion of the organotin halides produced are employed as intermediates for preparing organotin derivatives that are useful as catalysts, herbicides, insectides, anti-microbial agents and for a variety of other applications in the agricultural, coating and chemical industries. When applied to heated glassware such as bottles and other containers, organotin halides decompose to yield an adherent coating of stannic oxide on the glass. This is often the first step in forming conductive, protective or decorative coatings on glass containers.
During the preparation and use of organotin halides, these compounds are often present as a solution or emulsion in water. Since the organotin compounds are relatively costly and may exhibit significant mammalian toxicity, an attempt is usually made to recover substantially all of these compounds from their aqueous solutions. In addition it is often desirable to recover the halides in a relatively pure form. This is particularly true during the preparation of these compounds, at which time they are usually combined with other organotin compounds which must be absent from the final product. A conventional method for recovering water-soluble organotin halides from an aqueous solution is by distillation whereby a portion or all of the water is boiled off, leaving a residue of the desired organotin compound. If the organotin compound is present in combination with other water-soluble organotin compounds, the boiling points may differ sufficiently to permit separation of the organotin compounds by fractional distillation once the water is removed. Distillation may not be possible without significant loss of product if the boiling point of the organotin halide is close to that of water, if the organotin compound and water form an azeotropic mixture or if the organotin compound decomposes to any appreciable extent when heated to the temperature required to remove the water. Even if the desired separation can be effected by distillation, the process requires expensive equipment and considerable amounts of energy in the form of heat. In addition, the recovered organotin compound may require further processing to attain the desired level of purity. Organotin halides that melt above ambient temperature can often be purified by recrystallization, however this requires an additional process step and the use of organic solvents. Since many of the organic solvents are flammable and/or volatile, they present a hazard to the safety and health of personnel handling these materials in addition to increasing manufacturing costs.
An objective of this invention is to provide a simple, relatively low cost method for recovering some of the water-soluble organotin halides in relatively pure form and high yield from aqueous solutions containing these compounds. It has now been found that this objective can be achieved by adding to the aqueous solution containing certain organotin halides a strong inorganic electrolyte in an amount sufficient to form a partially or completely saturated solution at ambient temperature and then heating the resultant solution as necessary to effect separation of the organotin halide.